Semiconductor devices and methods of manufacture thereof

ABSTRACT

Methods of forming transistors and structures thereof are disclosed. A preferred embodiment comprises a semiconductor device including a workpiece, a gate dielectric disposed over the workpiece, and a thin layer of conductive material disposed over the gate dielectric. A layer of semiconductive material is disposed over the thin layer of conductive material. The layer of semiconductive material and the thin layer of conductive material comprise a gate electrode of a transistor. A source region and a drain region are formed in the workpiece proximate the gate dielectric. The thin layer of conductive material comprises a thickness of about 50 Angstroms or less.

This application is a divisional of patent application Ser. No. 11/300,050, entitled “Semiconductor Devices and Methods of Manufacture Thereof,” filed on Dec. 14, 2005, now U.S. Pat. No. 7,495,290 which application is incorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to the following co-pending and commonly assigned U.S. patent applications: Ser. No. 11/187,197, filed on Jul. 21, 2005, entitled, “CMOS Transistors With Dual High-k Gate Dielectric and Methods of Manufacture Thereof”; and Ser. No. 10/870,616, filed on Jun. 17, 2004, entitled, “CMOS Transistor With Dual High-k Gate Dielectric and Method of Manufacture Thereof”; which applications are hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to the fabrication of semiconductor devices, and more particularly to the fabrication of transistors.

BACKGROUND

Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment, as examples. Semiconductor devices are typically fabricated by sequentially depositing insulating (or dielectric) layers, conductive layers, and semiconductive layers of material over a semiconductor substrate, and patterning the various layers using lithography to form circuit components and elements thereon.

A transistor is an element that is utilized extensively in semiconductor devices. There may be millions of transistors on a single integrated circuit (IC), for example. A common type of transistor used in semiconductor device fabrication is a metal oxide semiconductor field effect transistor (MOSFET), as an example.

Early MOSFET processes used one type of doping to create either positive or negative channel transistors. More recent designs, referred to as complimentary MOS (CMOS) devices, use both positive and negative channel devices, e.g., a positive channel metal oxide semiconductor (PMOS) transistor and a negative channel metal oxide semiconductor (NMOS) transistor arranged in complimentary configurations. An NMOS device negatively charges so that the transistor is turned on or off by the movement of electrons, whereas a PMOS devices involves the movement of electron vacancies. While the manufacture of CMOS devices requires more manufacturing steps and more transistors, CMOS devices are advantageous because they utilize less power, and the devices may be made smaller and faster.

There is a trend towards reducing the size of semiconductor devices in the industry. However, manufacturing challenges are introduced as the dimensions of the various material layers are decreased, and patterning the material layers in smaller and smaller dimensions is also problematic.

Another trend in the semiconductor industry is the use of high dielectric constant (k) dielectric materials for a gate dielectric material of transistors. High k dielectric materials have a dielectric constant of greater than about 4.0, e.g., greater than the dielectric constant of SiO₂, which has been used as a gate dielectric material of transistors for many years. Because of their increased insulative properties, high k dielectric materials may be used in thinner dimensions than SiO₂ in transistor applications. However, the use of high k dielectric materials results in undesirable effects on the transistor, to be described further herein.

Thus, what are needed in the art are improved transistors and methods of manufacture thereof.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention which provide novel methods of manufacturing transistors and structures thereof.

In accordance with a preferred embodiment of the present invention, a semiconductor device includes a workpiece, a gate dielectric disposed over the workpiece, and a thin layer of conductive material disposed over the gate dielectric. A layer of semiconductive material is disposed over the thin layer of conductive material. The layer of semiconductive material and the thin layer of conductive material comprise a gate electrode of a transistor. A source region and a drain region are formed in the workpiece proximate the gate dielectric. The thin layer of conductive material comprises a thickness of about 50 Angstroms or less.

The foregoing has outlined rather broadly the features and technical advantages of embodiments of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of embodiments of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a prior art transistor that exhibits a polysilicon (poly) depletion effect;

FIGS. 2 through 5 show cross-sectional views of a method of fabricating a transistor in accordance with a preferred embodiment of the present invention;

FIG. 6 illustrates a cross-sectional view of a CMOS device in accordance with an embodiment of the present invention;

FIG. 7 is a graph showing the effect of a thin layer of conductive material comprising TiN on the effective work function of a transistor for three types of gate dielectric materials;

FIG. 8 shows an embodiment of the present invention implemented in a FinFET device;

FIG. 9 shows an embodiment of the present invention implemented in a tri-gate FET device; and

FIGS. 10 through 13 show cross-sectional views of a method of fabricating a transistor in accordance with an embodiment of the present invention.

Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the preferred embodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

FIG. 1 is a cross-sectional view of a prior art transistor 100 that exhibits a poly depletion effect. Conventional transistors 100 are formed by depositing a gate dielectric material 104 comprising SiO₂ or SiN over a workpiece 102, and depositing a gate material 106 comprising polysilicon over the gate dielectric material 104. The gate material 106 and the gate dielectric material 104 are patterned into the shape of a gate 106 and gate dielectric 104 of a transistor using lithography. Sidewall spacers 108 are formed on the sidewalls of the gate 106 and the gate dielectric 104, and a source region S and drain region D are formed in the workpiece 102 proximate the gate 106 and gate dielectric 104, as shown.

If the transistor 100 comprises an NMOS transistor, the polysilicon gate 106 is doped with an N type dopant such as arsenic. If the transistor 100 comprises a PMOS transistor, the polysilicon gate 106 is doped with a P type dopant such as boron. However, boron diffuses easily into adjacent material layers. Thus, doping the gate 106, e.g., with boron, can present problems, because if too high of a dopant concentration is implanted into the gate 106, the dopant can penetrate the gate dielectric 104 and also diffuse into the workpiece 102 beneath the gate dielectric 104. If the dopant enters the gate dielectric 104, leakage issues can be caused, and if the dopant enters the workpiece 102, the off state leakage current may be increased, which increases the power consumption. This problem is referred to in the art as “boron penetration,” where the boron dopant species does not remain in the gate 106, but rather, diffuses to other areas (104, 102) where the dopant does not belong, causing device 100 problems.

To avoid this problem in the prior art, the amount of dopants implanted into the gate 106 are sometimes limited; e.g., the amount of dopant, e.g., boron, is reduced to a minimum. However, this can result in carrier depletion in the polysilicon gate 106. For example, if the device 100 comprises a PMOS transistor 100, the PMOS transistor 100 is typically operated by applying a negative voltage to the polysilicon gate 106. The majority carrier when the gate 106 is doped with boron is a P type carrier or holes. When a negative voltage is applied to the polysilicon gate 106, holes are depleted in a region 110 close to or proximate the gate dielectric 104, as shown in FIG. 1. The depletion region 110 may extend from the interface of the gate dielectric 104 to the gate 106 (e.g., where the gate dielectric 104 abuts the gate 106) upwardly into the gate 106 by an amount d₁ of about 200 to 500 Angstroms, for example. Carriers (e.g., the holes) are depleted in the depletion region 110, which increases the capacitance effective thickness (CET) of the transistor 100, e.g., by about 4 to 5 Angstroms. The depletion region 110 comprises a polysilicon (poly) depletion effect that degrades the electrical characteristics of the transistor 100 and increases the effective gate dielectric 104 thickness. For example, the effective electric thickness of the gate dielectric 104 may be increased by the thickness d₁ of the poly depletion region 110.

The CET is often represented in the art as T_(inv), which is the inversion oxide thickness of the gate dielectric 104. The use of polysilicon as a gate 106 material exhibits a poly depletion effect which increases T_(inv) by about 4 to 5 Angstroms. In technologies having a very small minimum feature size, e.g., such as about 45 nm or less, a 5 Angstroms increase in T_(inv) may comprise approximately a 25% performance reduction, e.g., if the targeted T_(inv) is 20 Angstroms, for example.

The poly depletion effect is caused by a reduction in the amount of carriers in the polysilicon gate 106 when the device 100 is turned on; e.g., a voltage “high” for an NMOS device 100 and a voltage “low” for a PMOS device 100. During operation of the device 100, the high voltage applied to the NMOS device (e.g., which comprises n+ doped polysilicon in this case) drives the carriers (electrons) out of the polysilicon gate 106/gate dielectric 104 interface (e.g., at depletion region 108) and causes the depletion region 108 to form at the interface of the gate 106 to the gate dielectric 104.

Thus, what are needed in the art are gate electrode materials and transistor designs that do not exhibit a poly depletion effect.

In electronics, the “work function” is the energy, usually measured in electron volts, needed to remove an electron from the Fermi level to a point an infinite distance away outside the surface. Work function is a material property of any material, whether the material is a conductor, semiconductor, or dielectric. The work function of a semiconductor can be changed by doping the semiconductor material. For example, undoped polysilicon has a work function of about 4.5 eV, whereas polysilicon doped with boron has a work function of about 5.0 eV. The work function of a semiconductor or conductor directly affects the threshold voltage of a transistor when the material is used as a gate electrode.

In prior art transistor devices such as device 100 shown in FIG. 1 utilizing SiO₂ or SiN as the gate dielectric material 104, the work function of the transistor 100 could be changed or tuned by doping the polysilicon used for the gate electrode material 106. As mentioned earlier, there is a trend in the semiconductor industry towards the use of high k dielectric materials as gate dielectrics, to achieve a reduced gate dielectric 104 thickness. However, high-k gate dielectric materials such as hafnium-based dielectric materials exhibit Fermi-pinning, which is caused by the interaction of the high-k gate dielectric materials with adjacent materials, e.g., the gate material. When used as a gate dielectric, high k gate dielectric materials pin or fix the work function, so that doping the polysilicon gate material does not change the work function of the transistor, resulting in the inability to tune the work function to achieve a desired threshold voltage.

In a CMOS device, it is desirable for the PMOS transistor and the NMOS transistor to have symmetric threshold voltages V_(t). For example, the NMOS transistor should have threshold voltage V_(tn) of about +0.2 to +5 V, and the PMOS transistor should have a threshold voltage V_(tp) of substantially the same negative value, e.g., about −0.2 to −5 V. However, if a high k dielectric material is used as a gate dielectric material of a CMOS device, symmetric threshold voltages may not be achievable, because the work function cannot be tuned by adjusting the doping of the polysilicon gate material.

Thus, what are needed in the art are transistor devices comprising traditional gate dielectric materials such as SiO₂ and SiON that do not exhibit a poly depletion effect, rather requiring the use of high k dielectric materials.

What are also needed in the art are transistor devices comprising high k dielectric materials that have a tunable work function and therefore have the ability to achieve symmetric threshold voltages V_(t) for PMOS and NMOS transistors of CMOS devices.

The present invention will be described with respect to preferred embodiments in a specific context, namely in the formation of gate electrodes of transistors. Embodiments of the invention may also be applied, however, to other applications where other electrodes or conductive features are used.

Embodiments of the present invention achieve technical advantages by providing novel transistor devices and methods of manufacturing thereof. A thin layer of conductive material is deposited over a gate dielectric of a transistor. A layer of semiconductive material is deposited over the thin layer of conductive material. The thin layer of conductive material provides carriers for the thin layer of conductive material, preventing a poly depletion effect. The thin layer of conductive material is sufficiently thin enough to avoid affecting the work function of the transistor, e.g., the layer of semiconductive material has a dominant effect on the work function.

FIGS. 2 through 5 show cross-sectional views of a method of fabricating a semiconductor device 200 in accordance with a preferred embodiment of the present invention. Like numerals are used for the various elements that were described in FIG. 1.

Referring to FIG. 2, first, a workpiece 202 is provided. The workpiece 202 may include a semiconductor substrate comprising silicon or other semiconductor materials covered by an insulating layer, for example. The workpiece 202 may also include other active components or circuits, not shown. The workpiece 202 may comprise silicon oxide over single-crystal silicon, for example. The workpiece 202 may include other conductive layers or other semiconductor elements, e.g., transistors, diodes, etc. Compound semiconductors, GaAs, InP, Si/Ge, or SiC, as examples, may be used in place of silicon. The workpiece 202 may comprise a silicon-on-insulator (SOI) substrate, for example.

A gate dielectric 204 is formed over the workpiece 202. The gate dielectric 204 is also referred to herein as a gate dielectric material. The gate dielectric 204 preferably comprises a thickness of about 50 Angstroms or less, for example, although alternatively, the gate dielectric 204 may comprise other dimensions. The gate dielectric 204 may be formed using chemical vapor deposition (CVD), atomic layer deposition (ALD), metal organic chemical vapor deposition (MOCVD), physical vapor deposition (PVD), or jet vapor deposition (JVD), as examples, although alternatively, other deposition methods may also be used to form the gate dielectric 204.

In some embodiments, the gate dielectric 204 preferably comprises an insulating material comprising Si combined with O, N, or both O and N. e.g., SiO₂, SiON, or Si₃N₄, as examples. The gate dielectric 204 preferably comprises a dielectric constant of about 3.9 to about 7.8 in these embodiments, for example. Using conventional dielectric materials such as SiO₂, SiON, or Si₃N₄ as a gate dielectric 204 is an advantage because the manufacturing processes for working with these materials in transistor devices are mature, for example. Furthermore, Fermi-pinning is avoided by the use of these materials as the gate dielectric 204, for example. The gate dielectric 204 may comprise one layer or multiple layers of these materials.

In other embodiments, the gate dielectric 204 may comprise a high k dielectric material having a dielectric constant of about 4.0 or greater, in one embodiment, for example. In these embodiments, the gate dielectric 204 may comprise HfO₂, HfSiO_(X), Al₂O₃, ZrO₂, ZrSiO_(X), Ta₂O₅, La₂O₃, SiO₂, TiO₂, CeO₂, Bi₄Si₂O₁₂, WO₃, Y₂O₃, LaAlO₃, BST (Ba_((a-x))Sr_(x)TiO₃), PST (PbSc_(x)Ta_((1-a))O₃), nitrides thereof, Si_(x)N_(y), SiON, HfAlO_(x), HfAlO_(x)N_(1-x-y), ZrAlO_(x), ZrAlO_(x)N_(y), SiAlO_(x), SiAlO_(x)N_(1-x-y), HfSiAlO_(x), HfSiAlO_(x)N_(y), ZrSiAlO_(x), ZrSiAlO_(x)N_(y), PZN (PbZn_(x)Nb_((1-x))O₃), PZT (PbZr_(x)Ti_((1-x))O₃), PMN (PbMg_(x)Nb_((1-x))O₃), combinations thereof, or multiple layers thereof, as examples, although alternatively, the gate dielectric material 204 may comprise other high k dielectric materials or other dielectric materials.

A thin layer of conductive material 220 is formed over the gate dielectric 204, as shown in FIG. 3. The thin layer of conductive material 220 preferably comprises a nitride of a metal having a thickness d₂ of about 50 Angstroms or less. In some embodiments, the thin layer of conductive material 220 preferably comprises a thickness d₂ of less than about 25 Angstroms, in some embodiments, for example. The thin layer of conductive material 220 preferably comprises a material and a thickness d₂ such that the thin layer of conductive material 220 has a substantially negligible effect on a work function of the transistor 230 to be formed, as shown in FIG. 5, for example, so that a subsequently-deposited layer of semiconductive material (see layer 206 in FIG. 4) has a dominant effect on the work function of the transistor 230.

The thin layer of conductive material 220 is preferably formed by PVD, for example. In some embodiments, the thin layer of conductive material 220 is preferably formed using one or more cycles of ALD, for example. The thin layer of conductive material 220 preferably comprises TiN, TaN, WN, MoN, or HfN, as examples, although other conductive materials may also be used. In some embodiments, the thin layer of conductive material 220 preferably comprises a mid-gap metal, for example. The thin layer of conductive material 220 provides carriers (e.g., holes or electrons) for current flow when a voltage is applied to the gate electrode 206/220 of the transistor 230 (see FIG. 5), to be described further herein.

A layer of semiconductive material 206 is formed over the thin layer of conductive material 220, as shown in FIG. 4. The layer of semiconductive material 206 preferably comprises polysilicon, and may alternatively comprise amorphous silicon or other semiconductor materials, as examples. The layer of semiconductive material 206 preferably comprises a thickness of about 5,000 Angstroms or less, for example.

The layer of semiconductive material 206, the thin layer of conductive material 220, and the gate dielectric 204 are patterned using lithography into the shape of a transistor device 230, as shown in FIG. 5. For example, the patterned layer of semiconductive material 206, the thin layer of conductive material 220, and the gate dielectric 204 may comprise a square or rectangular shape when viewed from a top view (not shown in the drawings). The layer of semiconductive material 206, the thin layer of conductive material 220, and the gate dielectric 204 may be patterned by depositing a layer of photoresist and/or hard mask (not shown) over the layer of semiconductive material 206, patterning the layer of photoresist and/or hard mask using a lithography mask, and using the layer of photoresist and/or hard mask as a mask to pattern or etch away exposed portions of the layer of semiconductive material 206, the thin layer of conductive material 220, and the gate dielectric 204, for example.

The manufacturing process is then continued to complete the transistor device 230, as shown in FIG. 5. For example, an insulating material 208 may be deposited over the patterned layer of semiconductive material 206, the thin layer of conductive material 220, the gate dielectric 204, and exposed portions of the workpiece 202, and the insulating material 208 may be anisotropically etched to form sidewall spacers 208 on the sidewalls of the patterned layer of semiconductive material 206, the thin layer of conductive material 220, and the gate dielectric 204, as shown. The spacers 208 may comprise an insulating material such as an oxide, nitride, or combinations thereof, as examples. Exposed portions, and optionally, the semiconductive material 206 may be implanted with dopants, to form source regions S and drain regions D in the top surface of the workpiece 202 proximate the gate electrode 206/220 (e.g., gate G) and gate dielectric 204, and also to increase the conductivity of the gate 206.

The work function of the gate G is preferably established by the layer of semiconductive material 206, rather than by the thin layer of conductive material 220, according to preferred embodiments of the present invention. The thin layer of conductive material 220 preferably has no effect or a negligible effect on the work function of the transistor 230. The work function of the transistor 230 may be set to be about 4.0 eV to about 5.2 eV, depending on the amount of and type of dopants implanted into the semiconductive material 206, for example. Areas of the semiconductor device 200 may be masked while the semiconductive material 206 is implanted with a desired level of dopant species, for example (not shown). The transistor device 230 may comprise a PMOS transistor or an NMOS transistor, as examples.

Only one transistor 230 is shown in FIGS. 2 through 5; alternatively, many transistors 230 may be manufactured simultaneously in accordance with embodiments of the present invention.

Embodiments of the present invention may be implemented in a CMOS device 300, as shown in FIG. 6. Like numerals are used for the various elements that were described for the embodiment shown in FIGS. 2 through 5. To avoid repetition, each reference number shown in FIG. 6 is not described again in detail herein. Rather, similar materials x02, x04, x06, x20, etc., are preferably used for the various material layers shown as were described for FIGS. 2 through 5, where x=2 in FIGS. 2 through 5, and x=3 in FIG. 6. As an example, the preferred and alternative materials and dimensions described for the thin layer of conductive material 220 in the description for FIGS. 2 through 5 are preferably also used for the thin layer of conductive material 320 a and 320 b shown in FIG. 6.

FIG. 6 illustrates a cross-sectional view of a CMOS device 300 in accordance with an embodiment of the present invention. The CMOS device 300 includes a PMOS transistor 350 a and an NMOS transistor 350 b arranged in a complementary configuration. In a preferred embodiment, the gate dielectrics 304 a and 304 b are formed from a single layer of insulating material 304 a/304 b, the thin layers of conductive material 320 a and 320 b are formed from a single layer 320 a/320 b of thin conductive material, and the layers of semiconductive material 306 a and 306 b are formed from a single layer 306 a/306 b of semiconductive material. Advantageously, a first gate dielectric 304 a of the PMOS transistor 350 a and a second gate dielectric of the NMOS transistor 350 b comprise the same material and thickness in this embodiment, which minimizes the number of manufacturing process steps. Similarly, a first thin layer of conductive material 320 a of the PMOS transistor 350 a and a second thin layer of conductive material 320 b of the NMOS transistor 350 b comprise the same material and thickness in this embodiment, minimizing the number of manufacturing process steps.

The layer of semiconductive material 306 a of the PMOS transistor 350 a may be implanted with a first dopant species, e.g., comprising a p type dopant, while the NMOS transistor 350 b is masked. Likewise, the layer of semiconductive material 306 b of the NMOS transistor 350 b may be implanted with a second dopant species, e.g., comprising an n type dopant, while the PMOS transistor 350 a is masked. The dopant levels may be selected to establish the work function of the PMOS transistor 350 a and NMOS transistor 350 b, for example, because the thin layers of conductive material 320 a and 320 b, respectively, do not substantially affect the work function of the transistors 350 a and 350 b, for example. Advantageously, the work functions of the transistors 350 a and 350 b may be tuned using the dopant concentration levels to achieve a symmetric threshold voltage V_(tp) and V_(tn) for the CMOS device 300.

Before depositing the material layers 304 a/304 b, 320 a/320 b, and 306 a/306 b, the workpiece 302 may be doped with N type dopants and P type dopants, e.g., to form an N well and P well, respectively (not shown), in the workpiece 302 for the PMOS transistor 350 a and NMOS transistor 350 b, respectively. The workpiece 302 may then be cleaned using a pre-gate cleaning process to remove contaminants or native oxide from the top surface of the workpiece 302. The pre-gate treatment may comprise an HF, HCl, or an ozone based cleaning treatment, as examples, although the pre-gate treatment may alternatively comprise other chemistries.

Shallow trench isolation (STI) regions 340 may be formed between what will be active areas in the PMOS and NMOS regions of the workpiece 302. If the workpiece 302 comprises an SOI substrate 302, the shallow trench isolation region 340 may be formed by patterning a top layer of semiconductive material (not shown) of the workpiece 302, and filling the patterned second layer of semiconductive material with an insulating material such as silicon dioxide, although other materials may be used, for example.

FIGS. 10 through 13 show cross-sectional views of a method of fabricating a semiconductor device in accordance with an embodiment of the present invention.

Referring to FIG. 10, a first gate dielectric 304 a is formed over the workpiece in the first region and the second region. A first thin layer of conductive material 320 a is formed over the first gate dielectric 304 a. A first layer of semiconductive material 306 a is formed over the first thin layer of conductive material 320 a.

As illustrated in FIG. 11, a first layer of semiconductive material 306 a is removed from the second region of the workpiece. Subsequently, the first thin layer of conductive material 320 a and the first gate dielectric 304 a are removed from the second region of the workpiece.

Referring to FIG. 12, a second gate dielectric 304 b is formed over the workpiece in the first region and the second region. A second thin layer of conductive material 320 b is formed over the second gate dielectric 304 b. A second layer of semiconductive material 306 b is formed over the second thin layer of conductive material 320 b.

Referring to FIG. 13, the second layer of semiconductive material 306 b, the second thin layer of conductive material 320 b, and the second gate dielectric 304 b are removed from the first region of the workpiece.

As illustrated in FIG. 6, the first layer of semiconductive material 306 a, the first thin layer of conductive material 320 a, and the first gate dielectric 304 a are patterned to form a first transistor (e.g., PMOS transistor 350 a). Similarly, the second layer of semiconductive material 306 b, the second thin layer of conductive material 320 b, and the second gate dielectric 304 b are patterned to form the second transistor (e.g., NMOS transistor 350 b).

In other embodiments, the PMOS transistor 350 a may be first be formed, and the NMOS transistor 350 b may then be formed, or vice versa. One portion of the workpiece 302 may be covered with a mask while the insulating, conductive, and material layers are formed over the workpiece, and these materials may be removed in unwanted areas using a lift-off technique over the mask when removing the mask, for example. In this embodiment, the first gate dielectric 304 a of the PMOS transistor 350 a and the second gate dielectric 304 b of the NMOS transistor 350 b may comprise different materials and/or different thicknesses (not shown). Likewise, the first thin layer of conductive material 320 a of the PMOS transistor 350 a the second thin layer of conductive material 320 b of the NMOS transistor 350 b may comprise different materials and/or different thicknesses (not shown).

After the transistor 230 shown in FIG. 5 and the transistors 350 a and 350 b shown in FIG. 6 are formed, processing of the semiconductor devices 200 and 300 is then continued, such as forming insulating and conductive layers over the transistors 230, 350 a, and 350 b, as examples (not shown). For example, one or more insulating materials (not shown) may be deposited over the transistors 230, 350 a, and 350 b, and contacts may be formed in the insulating materials in order to make electrical contact with the top surface of the semiconductive material 206, 306 a, and 360 b of the gate electrodes and source and/or drain regions S and D. Additional metallization and insulating layers may be formed and patterned over the top surface of the insulating material and contacts. A passivation layer (not shown) may be deposited over the insulating layers of the transistors 230, 350 a, and 350 b. Bond pads (also not shown) may be formed over the contacts, and a plurality of the semiconductor devices 200 and 300 may then be singulated or separated into individual die. The bond pads may be connected to leads of an integrated circuit package (not shown) or other die, for example, in order to provide electrical contact to the transistors 230, 350 a, and 350 b of the semiconductor devices 200 and 300.

The novel thin layer of conductive material 220, 320 a and 320 b described herein functions as a carrier source when the transistors 230, 350 a, and 350 b are in operation, avoiding a poly depletion effect. The novel transistors 230, 350 a, and 350 b have gates G, 306 a/320 a, and 306 b/320 b that include the thin layer of conductive material 220, 320 a, and 320 b, and also include a thicker layer of semiconductive material 206, 306 a, and 306 b that is doped with an appropriate dopant species for a particular type of transistor, e.g., PMOS transistors include a layer of semiconductive material 206 and 306 a that is preferably doped with a p type material, NMOS transistors include a layer of semiconductive material 206 and 306 b that is preferably doped with an n type material.

Advantageously, the gate dielectric 204, 304 a, and 304 b may comprise traditional gate dielectric materials, such as SiO₂, SiON, or Si₃N₄, so that the manufacturing process is easily implemented into existing manufacturing process flows. The thin layer of conductive material 220, 320 a, and 320 b is also easily implementable into existing manufacturing process flows: for example, if the thin layer of conductive material 220, 320 a, and 320 b comprises TiN, the thin layer of TiN may be formed by introducing a Ti precursor into a deposition chamber simultaneously while introducing a nitrogen-containing gas such as NH₃, for example.

Another advantage of embodiments of the present invention is the prevention of boron diffusion. For example, if the transistor 230, 350 a comprises a PMOS transistor and the layer of semiconductive material 206 and 306 a is implanted with boron, the thin layer of conductive material 220 and 320 a prevents boron from diffusing into the gate dielectric 204 and 304 a, for example. If the thin layer of conductive material 220 and 320 a comprises a nitride material, for example, nitride provides an excellent diffusion barrier for boron, for example.

FIG. 7 is a graph showing the effect of a thin layer of conductive material (e.g., 220, 320 a, or 320 b) comprising TiN on the effective work function of a transistor for three types of gate dielectric materials for an increasing number of ALD deposition cycles. For example, as the number of ALD deposition cycles is increased, the thin layer of conductive material 220 thickness is increased. Effective work function measurements in eV are shown at 462 for a gate dielectric 204 comprising SiO₂ and a thin layer of conductive material 220 comprising TiN, at 464 for a gate dielectric 204 comprising HfSiO_(x) and a thin layer of conductive material 220 comprising TiN, and at 466 for a gate dielectric 204 comprising HfO₂ and a thin layer of conductive material 220 comprising TiN. The thinner the thin layer of conductive material 220 is (e.g., fewer ALD cycles are used to form the thin layer of conductive material 220), the more the work function of the electrode stack (e.g., including the thin layer of conductive material 220 and the layer of semiconductive material 206) is controlled by the level of dopant in the layer of semiconductive material 206. For example, on the right side of the graph, e.g., at 400 cycles, the work function of the electrode stack is controlled by the thin layer of conductive material 220. Therefore, preferably, in accordance with embodiments of the present invention, the thin layer of conductive material 220 is preferably formed using about 50 cycles of ALD, as shown at 460. In another embodiment of the present invention, the thin layer of conductive material 220 is preferably formed using about 50 cycles of ALD or less, as shown at 468. In another embodiment, the thin layer of conductive material 220 is preferably formed using about 100 cycles or less of ALD, as examples.

In Table 1, measurements of the work function of a layer of semiconductive material 206 comprising n doped polysilicon formed on a gate dielectric 204 comprising SiO₂ are shown for various dimensions of the thin layer of conductive material 220.

TABLE 1 Thin layer of Thin layer of Thickness of thin layer of conductive conductive conductive material 220 (in material 220 material 220 Angstroms) comprising WN comprising MoN 0 4.03 4.04 5 4.27 4.30 10 4.22 4.20 15 4.21 4.17 20 4.26 — 25 — 4.14 100 4.60 4.52

In Table 2, measurements of the work function of a layer of semiconductive material 206 comprising p doped polysilicon formed on a gate dielectric 204 comprising SiO₂ are shown for various dimensions of the thin layer of conductive material 220.

TABLE 2 Thin layer of Thin layer of Thickness of thin layer of conductive conductive conductive material 220 (in material 220 material 220 Angstroms) comprising WN comprising MoN 0 5.14 5.20 5 4.67 4.71 10 4.57 4.64 15 4.53 4.55 20 — — 25 — 4.44 100 4.60 4.52

Comparing Table 2 to Table 1, it can be seen that the thinner the thin layer of conductive material 220 is, the greater the difference between the work function is, and lower work functions are achieved, illustrating that embodiments of the present invention are useful in CMOS applications. For example, referring to FIG. 6, if the gate dielectric 304 a and 304 b comprises SiO₂, the thin layer of conductive material 320 a and 320 b comprises about 5 Angstroms of MoN, and the layer of semiconductive material 306 a and 306 b comprises polysilicon doped with n type or p type dopants for the NMOS transistor 350 b and PMOS transistor 350 a, respectively, about a 400 mV work function difference is achieved (e.g., 4.71 eV in Table 2 less 4.3 eV in Table 1) between the NMOS and PMOS transistors 350 b and 350 a, which is a sufficient amount of work function difference for many CMOS low standby power (LSTP) device applications, for example. This embodiment may be particularly beneficial when the workpiece 202 comprises an SOI substrate, for example.

In Table 3, measurements of the work function of a layer of semiconductive material 206 comprising n doped polysilicon formed on a gate dielectric 204 comprising HfO₂ are shown for various dimensions of the thin layer of conductive material 220.

TABLE 3 Thin layer of Thin layer of Thickness of thin layer of conductive conductive conductive material 220 (in material 220 material 220 Angstroms) comprising WN comprising MoN 0 4.24 4.25 5 — 4.29 10 4.3 4.26 15 4.32 4.26 20 4.36 4.25

In Table 4, measurements of the work function of a layer of semiconductive material 206 comprising p doped polysilicon formed on a gate dielectric 204 comprising HfO₂ are shown for various dimensions of the thin layer of conductive material 220.

TABLE 4 Thin layer of Thin layer of Thickness of thin layer of conductive conductive conductive material 220 (in material 220 material 220 Angstroms) comprising WN comprising MoN 0 4.00 4.23 5 4.34 4.32 10 4.3 4.27 15 4.33 4.24 20 4.35 4.25

Comparing Table 4 to Table 3, it can be seen that for WN, the thinner the thin layer of conductive material 220 is, the greater the difference between the work function is, illustrating that embodiments of the present invention are useful in CMOS applications. WN is an effective material to use for the thin layer of conductive material 220, even when a high k dielectric material is used for the gate dielectric 204, in accordance with embodiments of the present application.

In accordance with some embodiments of the present invention, it may be advantageous to use a different high k dielectric material for a PMOS transistor 350 a (e.g., as shown in FIG. 6) than for an NMOS transistor 350 b of a CMOS device 300, as described in U.S. patent applications: Ser. No. 11/187,197, filed on Jul. 21, 2005, entitled, “CMOS Transistors With Dual High-k Gate Dielectric and Methods of Manufacture Thereof”; and Ser. No. 10/870,616, filed on Jun. 17, 2004, entitled, “CMOS Transistor With Dual High-k Gate Dielectric and Method of Manufacture Thereof”; which are incorporated herein by reference.

As an example, referring to Tables 3 and 4, if the gate dielectrics 304 a and 304 b both comprise HfO₂, the thin layer of conductive material 320 a and 320 b both comprise about 5 Angstroms of MoN, and the layer of semiconductive material 306 a and 306 b both comprise polysilicon doped with n type dopants for the NMOS transistor 350 b and doped with p type dopants for the PMOS transistor 350 a, then there would be less than a 50 mV difference in the work function between the NMOS transistor 350 b and the PMOS transistor 350 a (e.g., 4.32 in Table 4 less 4.29 in Table 3), which is not a sufficient work function difference for some CMOS applications. Therefore, in order to have a large enough work function separation for the PMOS transistor 350 a and NMOS transistor 350 b, it is preferable to use two different high-k dielectric materials for the gate dielectric 304 b and 304 a of the NMOS and PMOS transistors 350 b and 350 a, respectively.

For example, in one preferred embodiment, the gate dielectric 304 a of the PMOS transistor 350 a preferably comprises a first layer of HfSiO and a second layer of Al₂O₃ disposed over the first layer of HfSiO, and the gate dielectric 304 b of the NMOS transistor 350 b preferably comprises a single layer of HfSiO. The thin layer of conductive material 320 a and 320 b may comprise about 50 Angstroms or less of metal such as TiN, and the layer of semiconductive material 306 a and 306 b may comprise doped polysilicon. Using these gate stack structure materials, for example, a wider separation of work functions for the NMOS transistor 350 b and the PMOS transistor 350 a is achievable, which is needed in many CMOS applications, for example.

Advantages of embodiments of the invention include solving the poly depletion problem exhibited in many transistor designs. The T_(inv) can be reduced, resulting in improved device performance, while simultaneously reducing boron penetration. Traditional SiO₂ or SiON gate dielectric materials may be used, avoiding concerns of reliability problems that may be an issue with the use of some high-k dielectric materials, for example. In CMOS applications, a single thin layer of conductive material 320 a/320 b may be formed, avoiding integration issues with a dual metal scheme.

Embodiments of the present invention may be applied in many types of applications, and are particularly useful in transistor applications where low power and high speed are a concern. Embodiments of the present invention are also particularly useful in reduced scale or size transistor designs, for example.

Embodiments of the present invention also have useful application in multi-gate transistors, or multi-gate field effect transistors (MugFETs), as shown in FIGS. 8 and 9. Again, like numerals are used for the various elements that were described for the embodiments shown in FIGS. 2 through 5 and FIG. 6, and to avoid repetition, each reference number shown in FIGS. 8 and 9 is not described again in detail herein. Rather, similar materials x02, x04, x06, x20, etc., are preferably used for the various material layers shown as were described for FIGS. 2 through 5 and 6, where x=2 in FIGS. 2 through 5, x=3 in FIG. 6, x=5 in FIG. 8, and x=6 in FIG. 9.

MugFETs are an emerging transistor technology. A double gate transistor has two parallel gates that face each other and control the same channel region. A finFET is a vertical double gate device, wherein the channel comprises a vertical fin comprising a semiconductor material, which may be formed on a silicon-on-insulator (SOI) substrate, for example. The two gates of a finFET are formed on opposing sidewalls of the vertical fin. A tri-gate transistor has three gates that control the same channel region, e.g., the channel comprises the vertical fin, two of the gates are formed on the sides of the vertical fin, and a third gate is formed on the top of the fin. A finFET structure is similar to a tri-gate transistor, with the third gate being blocked by an insulating material or hard mask disposed on top of the fin, for example.

Two or more MugFETs may be formed in parallel to form an NMOS transistor 550 b and 650 b and PMOS transistor 550 a and 550 b of a CMOS MugFET device, as shown in FIGS. 8 and 9. FIG. 8 shows an embodiment of the present invention implemented in a FinFET device, wherein two gate electrodes 520 a/506 a and 520 b/506 b are disposed on the sidewalls of each fin structure 505. FIG. 9 shows an embodiment of the present invention implemented in a tri-gate FET, wherein two gate electrodes 620 a/606 a and 620 b/606 b are disposed on the sidewalls of each fin structure 605, and a third gate electrode 620 a/606 a and 620 b/606 b is disposed on the top of each fin structure 605. The novel thin layer of conductive material 520 a, 520 b, 620 a, and 620 b is formed between the gate dielectric 504 a, 504 b, 604 a, and 604 b, and the layer of semiconductive material 506 a, 506 b, 606 a, and 606 b, as shown.

To form the novel FinFET transistors 550 a and 550 b and Tri-gate FETs 650 a and 650 b shown in FIGS. 8 and 9, a workpiece 502 and 602 comprising an SOI substrate is provided. Referring to FIG. 8, the SOI substrate 502 includes a first layer of semiconductive material 501 that comprises a substrate, a buried insulating layer 503 or buried oxide layer disposed over the first layer of semiconductive material 501, and a second layer of semiconductive material 505 disposed over the buried insulating layer 503, for example. The second layer of semiconductor material 505 may comprise silicon (Si) having a thickness of about 100 nm, for example, although alternatively, the second layer of semiconductor material 505 may comprise other materials and dimensions.

A hard mask 509/511/513 is formed over the workpiece 502. The hard mask 509/511/513 comprises a first oxide layer 509 comprising about 5 nm or less of SiO₂ formed over the workpiece 502. A nitride layer 511 comprising about 20 nm of Si_(x)N_(y) is formed over the first oxide layer 509. A second oxide layer 513 comprising about 20 nm or less of SiO₂ is formed over the nitride layer 511. Alternatively, the hard mask 509/511/513 may comprise other materials and dimensions, for example.

The hard mask 509/511/513 is patterned using lithography, e.g., by depositing a layer of photoresist (not shown) over the hard mask 509/511/513, exposing the layer of photoresist to energy using a lithography mask, developing the layer of photoresist, and using the layer of photoresist as a mask to pattern the hard mask 509/511/513, for example. The hard mask 509/511/513, and optionally, also the layer of photoresist, are used as a mask to pattern the second layer of semiconductive material 505 of the workpiece 502 to form fin structures 505 in the layer of semiconductive material 505, as shown in FIG. 8. The buried insulating layer 503 may comprise an etch stop layer for the etch process of the second layer of semiconductive material 505, for example. A top portion of the buried insulating layer 503 may be removed during the etch process of the second layer of semiconductive material 505, as shown. For example, the buried insulating layer 503 may have a thickness of about 150 nm, and may be etched by about 15 nm or less.

The second layer of semiconductor material 505 of the SOI substrate or workpiece 502 forms vertical fins of semiconductor material 505 extending in a vertical direction away from a horizontal direction of the workpiece 502. The fin structures 505 will function as the channels of PMOS and NMOS transistors 550 a and 550 b. The fin structures 505 may also be formed on a bulk substrate 502, for example. The fin structures 505 have a thickness that may comprise about 50 nm or less, as an example, although alternatively, the fins 505 may comprise other dimensions. For example, the thickness of the fin structures 505 may comprise about 5 to 60 nm, or less, in some applications. As another example, the thickness of the fin structures 505 may be larger, having a thickness of about 100 to 1,000 nm, as another example. The thickness of the fin structures 505 may vary as a function of the channel doping and other dimensions of the fin structures 505, as examples, although other parameters may also have an effect on the determination of the dimension of the fin structure 505 thickness.

The fin structures 505 have a height that is substantially equivalent to the thickness of the second layer of semiconductor material 505, for example. Only two fin structures 505 are shown for the PMOS transistor 530 a and the NMOS transistor 530 b; however, there may be many fin structures 505, e.g., about 1 to 200 fin structures 505 for each PMOS and NMOS transistor 530 a and 530 b, respectively, although alternatively, other numbers of fin structures 505 may be used.

After the fin structures 505 are formed, then the gate dielectric 504 a and 504 b, the thin layer of conductive material 520 a and 520 b, and the layer of semiconductive material 506 a and 506 b are deposited sequentially over the fin structures 505, as shown in FIG. 8, and the manufacturing process is continued, as previously described herein, for example. A first gate electrode is formed on a left sidewall of each fin structure 505 comprised of the thin layer of conductive material 520 a and the layer of semiconductive material 506 a, and a second gate electrode is formed on a right sidewall of each fin structure 505 comprised of the thin layer of conductive material 520 a and the layer of semiconductive material 506 a, for example.

In FIG. 9, to form Tri-gate FETs 650 a and 650 b, before the gate dielectric 604 a and 604 b is deposited, the hard mask (e.g., hard mask 509/511/513 shown in FIG. 8) is removed. A first gate electrode is formed on a left sidewall of each fin structure 605 comprised of the thin layer of conductive material 620 a and the layer of semiconductive material 606 a, and a second gate electrode is formed on a right sidewall of each fin structure 605 comprised of the thin layer of conductive material 620 a and the layer of semiconductive material 606 a, for example. A third gate electrode is formed on the top surface of each fin structure 605 comprised of the thin layer of conductive material 620 a and the layer of semiconductive material 606 a.

Although embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, it will be readily understood by those skilled in the art that many of the features, functions, processes, and materials described herein may be varied while remaining within the scope of the present invention. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

1. A method of manufacturing a semiconductor device, the method comprising: forming a gate dielectric material over a workpiece; forming a thin layer of conductive material over the gate dielectric material, the thin layer of conductive material physically contacting the gate dielectric material; forming a layer of semiconductive material over the thin layer of conductive material; and patterning the layer of semiconductive material, the thin layer of conductive material, and the gate dielectric material to form at least one PMOS transistor, wherein the gate dielectric material comprises a gate dielectric of the at least one PMOS transistor, wherein the layer of semiconductive material and the thin layer of conductive material comprise a gate electrode of the at least one PMOS transistor, and wherein forming the thin layer of conductive material comprises forming a layer of conductive material comprising a thickness such that the thin layer of conductive material has a substantially negligible effect on a work function of the gate electrode of the at least one PMOS transistor.
 2. The method according to claim 1, wherein forming the thin layer of conductive material comprises physical vapor deposition (PVD) of a metal nitride.
 3. The method according to claim 1, further comprising implanting a dopant species into the layer of semiconductive material, wherein a work function of the gate electrode of the at least one PMOS transistor is established by the layer of semiconductive material.
 4. The method according to claim 1, further comprising forming a source region and a drain region in the workpiece proximate the gate dielectric and the gate electrode of the at least one PMOS transistor.
 5. A method of manufacturing a semiconductor device, the method comprising: providing a workpiece; forming a first transistor in a first region of the workpiece, the first transistor comprising a first gate dielectric disposed over the workpiece and a first gate electrode disposed over the first gate dielectric, the first gate electrode comprising a first thin layer of conductive material and a first layer of semiconductive material disposed over the first thin layer of conductive material, wherein the first thin layer of conductive material comprises a thickness of about 50 Angstroms or less, wherein a first work function of the first gate electrode of the first transistor is established by the first layer of semiconductive material; and forming a second transistor in a second region of the workpiece proximate the first region of the workpiece, the second transistor comprising a second gate dielectric disposed over the workpiece and a second gate electrode disposed over the second gate dielectric, the second gate electrode comprising a second thin layer of conductive material and a second layer of semiconductive material disposed over the second thin layer of conductive material, wherein the second thin layer of conductive material comprises a thickness of about 50 Angstroms or less, wherein a second work function of the second gate electrode of the second transistor is established by the second layer of semiconductive material.
 6. The method according to claim 5, wherein forming the first transistor comprises forming a positive channel metal oxide semiconductor (PMOS) transistor, wherein forming the second transistor comprises forming a negative channel metal oxide semiconductor (NMOS) transistor, and wherein the PMOS transistor and the NMOS transistor comprise a complementary metal oxide semiconductor (CMOS) device.
 7. The method according to claim 6, further comprising doping the first layer of semiconductive material with a p type dopant species, and doping the second layer of semiconductive material with an n type dopant species.
 8. The method according to claim 5, wherein the first gate dielectric and the second gate dielectric comprise the same material, different materials, the same thickness, or different thicknesses, and wherein the first thin layer of conductive material and the second thin layer of conductive material comprise the same material, different materials, the same thickness, or different thicknesses.
 9. The method according to claim 5, wherein forming the first transistor and forming the second transistor comprise: forming the first gate dielectric over the workpiece in the first region and the second region; forming the first thin layer of conductive material over the first gate dielectric; forming the first layer of semiconductive material over the first thin layer of conductive material; and patterning the first layer of semiconductive material, the first thin layer of conductive material, and the first gate dielectric to form the first transistor and the second transistor, wherein the second gate dielectric of the second transistor comprises the first gate dielectric, wherein the second thin layer of conductive material comprises the first thin layer of conductive material, and wherein the second layer of semiconductive material comprises the first layer of semiconductive material.
 10. A method of manufacturing a semiconductor device, the method comprising: providing a workpiece; forming a first transistor in a first region of the workpiece, the first transistor comprising a first gate dielectric disposed over the workpiece and a first gate electrode disposed over the first gate dielectric, the first gate electrode comprising a first thin layer of conductive material and a first layer of semiconductive material disposed over the first thin layer of conductive material, wherein the first thin layer of conductive material comprises a thickness of about 50 Angstroms or less, wherein a first work function of the first gate electrode of the first transistor is established by the first layer of semiconductive material; and forming a second transistor in a second region of the workpiece proximate the first region of the workpiece, the second transistor comprising a second gate dielectric disposed over the workpiece and a second gate electrode disposed over the second gate dielectric, the second gate electrode comprising a second thin layer of conductive material and a second layer of semiconductive material disposed over the second thin layer of conductive material, wherein the second thin layer of conductive material comprises a thickness of about 50 Angstroms or less, wherein a second work function of the second gate electrode of the second transistor is established by the second layer of semiconductive material, and wherein forming the first transistor and forming the second transistor comprise: forming the first gate dielectric over the workpiece in the first region and the second region; forming the first thin layer of conductive material over the first gate dielectric; forming the first layer of semiconductive material over the first thin layer of conductive material; removing the first layer of semiconductive material, the first thin layer of conductive material, and the first gate dielectric from the second region of the workpiece; forming the second gate dielectric over the workpiece in the first region and the second region; forming the second thin layer of conductive material over the second gate dielectric; forming the second layer of semiconductive material over the second thin layer of conductive material; removing the second layer of semiconductive material, the second thin layer of conductive material, and the second gate dielectric from the first region of the workpiece; patterning the first layer of semiconductive material, the first thin layer of conductive material, and the first gate dielectric to form the first transistor; and patterning the second layer of semiconductive material, the second thin layer of conductive material, and the second gate dielectric to form the second transistor.
 11. A method of manufacturing a complimentary metal oxide semiconductor (CMOS) device, the method comprising: forming a PMOS transistor in a first region of a semiconductor body, the PMOS transistor including a first gate dielectric disposed over the semiconductor body and a first gate electrode disposed over the first gate dielectric, the first gate electrode comprising a first thin layer of conductive material and a first layer of semiconductive material disposed over the first thin layer of conductive material, wherein the first thin layer of conductive material comprises a first thickness such that the first thin layer of conductive material has a substantially negligible effect on a first work function of the PMOS transistor, and wherein the first work function is established by the first layer of semiconductive material; and forming an NMOS transistor in a second region of the semiconductor body proximate the first region, the NMOS transistor including a second gate dielectric disposed over the semiconductor body and a second gate electrode disposed over the first gate dielectric, the second gate electrode comprising a second thin layer of conductive material and a second layer of semiconductive material disposed over the second thin layer of conductive material, wherein the second thin layer of conductive material comprises a second thickness such that the second thin layer of conductive material has a substantially negligible effect on a second work function of the NMOS transistor, and wherein the second work function is established by the second layer of semiconductive material.
 12. The method according to claim 11, wherein the first layer of semiconductive material comprises polysilicon doped with a p type dopant, and wherein the second layer of semiconductive material comprises polysilicon doped with an n type dopant.
 13. The method according to claim 11, wherein the PMOS transistor and the NMOS transistor have substantially symmetric threshold voltages. 